One of the most efficient techniques for image compression is subband coding, such as wavelet transformation, where an image input signal is decomposed into several frequency subbands that are quantised and then coded for onward transmission or storage. Since this process is reversible, the original image signal can be reconstructed back from the coded subband information. An important practical problem in subband signal coding is that a high degree of complexity is introduced by decomposition/reconstruction when the signal length is substantial. This is especially critical for large images. The most commonly used approach to cope with this problem is to break an image into sub-images, known as blocks or tiles, of a smaller size and process them independently. This method, which is usually called non-overlapped tiling, can significantly reduce the complexity, but introduces very noticeable boundary artifacts in the regions of the edges of the tiles in the reconstructed image. Since a human eye is very sensitive to edges especially in uniform areas, these types of distortions are very noticeable and consequently degrade image quality dramatically. The problem of smooth boundary reconstruction is, of course, not limited to image coding, but exists in different aspects in many other applications.
Due to its practical importance, the problem of reducing boundary artifacts has been extensively investigated and described in many publications. One known approach is to use overlapping samples from adjacent tiles, as suggested by I1 Kye Eom, Yoon Soo Kim and Jae Ho Kim in an article entitled “A Block Wavelet Transform for Sub-image Coding/Decoding” published in SPIE Vol. 2669 at pages 169–177 and in U.S. Pat. No. 5,710,835. Another known approach is to employ a variety of post processing techniques at the image reconstruction stage, as described by B. Jeon and J. Jeong in an article entitled “Blocking Artifacts Reduction in Image Compression with Block Boundary Discontinuity Criterion” published in IEEE Trans. on Circuts and Systems for Video Tech. Vol 8, N3, 1998 at pages 345–367 and by J. K. Su and R. M. Mersereau in ana article entitled “Post-processing for Artifact Reduction in JPEG-compressed Images” published in ICASSP-95, Vol. 4 at pages 2363–2366. Unfortunately, most existing methods can only be applied in practical systems with great difficulty due to the substantial additional complexity which they introduce.
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